memory fragmentation with ghc-7.6.1

[John Lato]

Hello,

We've noticed that some applications exhibit significantly worse
memory usage when compiled with ghc-7.6.1 compared to ghc-7.4, leading
to out of memory errors in some cases.  Running one app with +RTS -s,
I see this:

ghc-7.4
 525,451,699,736 bytes allocated in the heap
  53,404,833,048 bytes copied during GC
      39,097,600 bytes maximum residency (2439 sample(s))
       1,547,040 bytes maximum slop
             628 MB total memory in use (0 MB lost due to fragmentation)

ghc-7.6
512,535,907,752 bytes allocated in the heap
  53,327,184,712 bytes copied during GC
      40,038,584 bytes maximum residency (2391 sample(s))
       1,456,472 bytes maximum slop
            3414 MB total memory in use (2744 MB lost due to fragmentation)

The total memory in use (consistent with 'top's output) is much higher
when built with ghc-7.6, due entirely to fragmentation.

I've filed a bug report
(http://hackage.haskell.org/trac/ghc/ticket/7257,
http://hpaste.org/74987), but I was wondering if anyone else has
noticed this?  I'm not entirely sure what's triggering this behavior
(some applications work fine), although I suspect it has to do with
allocation of pinned memory.

John L.